[2] Hayer worked as a parliamentary assistant for several members of the Senate of France, including Pierre Jarlier of the Union of Democrats and Independents (UDI) and then of LREM, and then for the MEP Jean Arthuis.

[11] In October 2021, Hayer and Marie-Pierre Vedrenne were appointed as co-chairs of President Emmanuel Macron's Renaissance electoral list in the European Parliament, replacing Stéphane Séjourné.

[13] In January 2024, Hayer was elected to succeed Séjourné as leader of Renew Europe, then the parliament’s third-largest political grouping.

[15] On February 29, 2024, Hayer confirmed being named head of the “Need for Europe” list of the presidential majority for the 2024 European elections.

[17][18][19] In March 2024, Hayer criticized Marine Le Pen and Viktor Orban for their relationships with Russian president Vladimir Putin, accusing them of appeasement.